The present invention relates to a method and an apparatus for supplying electric energy to a resistance welding gun, and more particularly to a method and an apparatus for supplying a welding current to a resistance welding gun mounted on a spot welding robot or the like, such that movement of the resistance welding gun will not be obstructed by a power cable and the current capacity will be increased.
To assemble an automotive body, it is general practice to join workpiece members in the form of thin metal sheets by spot welding. In recent years, such spot welding operation has been performed exclusively by spot welding robots.
A spot welding robot has a resistance welding gun comprising a pair of electrodes mounted on the distal ends of a pair of gun arms, respectively, movable toward and away from each other. Workpiece members such as thin metal sheets are gripped by the electrodes, and an electric current is passed between the electrodes across the thin metal sheets to join them to each other. Various types of resistance welding guns are known in the art. One type of resistance welding gun is swingable itself when the gun arms are moved toward and away from each other.
Such a resistance welding gun is disclosed in Japanese Laid-Open Utility Model Publication No. 57-49079 filed by the applicant. The disclosed resistance welding gun is shown in FIGS. 1 and 2 of the accompanying drawings.
As shown in FIGS. 1 and 2, the resistance welding gun, generally denoted at 2, has a support arm 6 fixed to a base 4 extending from a robot arm or the like and a bracket 8 swingably supported on the support arm 6 by means of a shaft 10. A fixed gun arm 12 is secured to the bracket 8, and an electrode 14 is mounted on the tip end of the fixed gun arm 12. An electrically conductive member 15 is fixed to the bracket 8 and has one end connected to the fixed gun arm 12. The other end of the conductive member 15 has a connector 15a projecting outwardly from the bracket 8. A compression spring 17 is connected between the connector 15a and the support arm 6.
A movable gun arm 16 of a bent shape is angularly movably supported on the bracket 8 in confronting relation to the fixed gun arm 12. An electrode 18 is mounted on the tip end of the movable gun arm 16, and a joint lever 20 is attached to the tail end of the movable gun arm 16. The joint arm 20 is operatively joined to a cylinder 22 which moves the movable gun arm 16 toward and away from the fixed gun arm 12. The cylinder 22 is fixedly mounted on the bracket 8 and has a piston 24 slidably disposed therein. A piston rod 26 coupled to and extending from the piston 24 has a distal end connected to the joint lever 20 through a pin 28. Another pin 30 is attached to the joint lever 20 in spaced relation to the pin 28. The pins 28, 30 ride in a J-shaped cam groove 32 defined in the bracket 8.
When the cylinder 22 is energized to move the piston 24 in the direction indicated by the arrow D in FIG. 1, the movable gun arm 16 is angularly moved in the direction indicated by the arrow B toward the fixed gun arm 12 while the pins 28, 30 are being guided in and along the cam groove 32. The compression spring 17 urges the bracket 8 to turn about the shaft 10 in the direction indicated by the arrow A, so that the fixed gun arm 12 is displaced toward a workpiece W. Finally, the workpiece W is gripped between the fixed gun arm 12 and the movable gun arm 16 as shown in FIG. 2.
The piston rod 26 has a step on a central region thereof, the step having a curved cam surface 34 as shown in FIGS. 1 and 2. When the piston 24 is displaced in the direction indicated by the arrow C in FIG. 2, the movable gun arm 16 is displaced in the direction indicated by the arrow A away from the workpiece W, and the cam surface 34 of the piston rod 26 abuts against a distal end 6a of the support arm 6. Since the piston 24 is allowed to move further in the direction indicated by the arrow C, the point at which the cam surface 34 engages the distal end 6a of the support arm 6 is shifted or displaced. The bracket 8 is turned as a whole in the direction indicated by the arrow B against the bias of the compression spring 17. As a result, the fixed gun arm 12 secured to the bracket 8 is moved away from the workpiece W, and resistance welding gun 2 is turned bodily in the direction indicated by the arrow B (see FIG. 1).
Since the resistance welding gun 2 is thus swung as a whole, it can be retracted away from the workpiece W. When the workpiece W is moved in a direction, for example, normal to the sheet of FIG. 1, to bring a new spot on the workpiece W between the electrodes 14, 18, the workpiece W may be kept out of physical interference with the fixed gun arm 12 and the movable gun arm 16.
To supply an electric current to the resistance welding gun 2, laminated copper sheet members 36a, 36b connected as a power cable to a power supply (not shown) are joined to the connector 15a of the conductive member 15 mounted on the bracket 8. The terminal end of the laminated copper sheet member 36a is electrically connected to the fixed gun arm 12 through the conductive member 15, whereas the terminal end of the laminated copper sheet member 36b is electrically connected to the movable gun arm 16 through an electrically conductive member 38 and a laminated copper sheet member 40.
When the resistance welding gun 2 is angularly moved into the position shown in FIG. 1, the laminated copper sheet members 36a, 36b secured to the resistance welding gun 2 are forcibly bent.
Modern automotive bodies are made of thin steel sheets which are treated for corrosion protection, e.g., galvanized. A welding current to be supplied for welding steel sheets of such automotive bodies is required to be large since it flows through the treated surfaces of the steel sheets. Power cables which carry such a welding current are large in cross section in order to increase their current capacity. The thick power cables are however difficult to flex freely, thus obstructing swinging movement of the resistance welding gun 2 out of interference with the workpiece W.
The poor flexibility of the power cable and slow movement of the resistance welding gun due to the large weight of the power cable are problematic not only during welding operation but also when retracting the welding gun from the workpiece for replacing the workpiece with a new one.